Calanus finmarchicus is one of the dominant copepod species in northern pelagic ecosystems, and its fecal pellet production is important for the carbon flux in this area. Size, shape and number of pellets produced are related to food quality. Females fed with the dinoflagellate Heterocapsa sp. produced thin, fragile pellets which dissolved soon after their release. In contrast, females fed with the diatom Thalassiosira weissflogii produced large, solid fecal pellets. Feeding on mixed cultures did also result in fragile pellets indicating a preference for dinoflagellates. Gut analysis, analysis of gonad development stages and egg production rate measurements showed that both algae were ingested and used for reproduction. If dissolvation of fecal pellets due to feeding on natural dinoflagellate assemblages occurs in the ocean, the carbon extruded by the copepods would be retained in surface waters and might be used by bacteria instead of sinking down to deeper water masses.